1. Field of the Invention
The present invention relates to a vehicle motion control system for controlling vehicle motion, and more particularly to the control system which performs a steering control by braking to restrain an excessive oversteer and excessive understeer which will occur during, for example, cornering, by applying a braking force to each wheel of the vehicle irrespective of depression of a brake pedal, and which performs an anti-skid control to prevent a wheel from being locked during braking, by controlling the braking force applied to the wheel.
2. Description of the Related Arts
Recently, a vehicle is provided with a braking force control system for controlling the braking force applied to the vehicle to perform an anti-skid control, a traction control, a front-rear braking force distribution control, and etc. In the U.S. Pat. No 4,898,431, for example, an apparatus for controlling vehicle motion through the use of a brake controlling system which compensates for the influence of lateral forces on the vehicle. The apparatus is constituted so as to control the braking force applied to the vehicle by the braking force control system in response to a comparison of a desired yaw rate with an actual yaw rate, thereby to improve the vehicle stability during the course of vehicle motion such as cornering. Consequently, a braking force is applied to each wheel irrespective of depression of a brake pedal, so that a so-called steering control by braking is performed to provide an oversteer restraining control and understeer restraining control.
With respect to a braking force control apparatus for applying a braking force to each wheel of a vehicle in response to a braking condition of the vehicle to correct the vehicle motion, it is proposed in a Japanese Patent Laid-open publication No. 7-117654 to correct an abnormal vehicle motion rapidly and properly by introducing a feedforward control in addition to a feedback control. In the apparatus, a feedforward control signal is output in response to a desired slip rate, and coupled with a feedback control signal which is output separately from the feedforward control signal, to produce an output signal to a hydraulic pressure control apparatus.
In the recent vehicle, also provided are an anti-skid control mode wherein a hydraulic braking pressure supplied to a wheel brake cylinder is controlled in response to a rotational speed of a wheel so as to control the braking force applied to the wheel, and the like modes, so that the anti-skid control mode and the like are combined with the mode of steering control by braking to provide the effective vehicle motion control system as a whole.
However, if the condition of the vehicle meets the requirements for starting the various control modes as described above, simultaneously, interference among the control modes will occur. For example, if the mode of steering control by braking and the anti-skid control mode are set for a single wheel simultaneously, the interference between those modes will occur. This interference occurs because the mode of steering control by braking is a control mode for applying the braking force to a wheel to be controlled, while the anti-skid control mode is a control mode for reducing the braking force applied to a wheel which tends to be locked. In this case, the anti-skid control mode generally takes priority over the other control mode to provide a so-called safeguard. In the case where the vehicle motion is in a zone for controlling the mode of steering control by braking and the anti-skid control mode simultaneously, however, even if the control mode to be performed is not the anti-skid control mode but the mode of steering control by braking, the anti-skid control is initiated, so that the mode of steering control by barking will be delayed.
Therefore, when plural control modes are set for the wheel to be controlled, it is necessary to provide an appropriate pressure mode until the control mode to be performed is completed, without giving priority to the anti-skid control mode automatically, unless the anti-skid control operation will be performed improperly. Or, when plural maps are provided to set the plural control modes, and when a hydraulic pressure mode is set for each control mode, a large capacity of memory in a computer will be required to execute those modes. Therefore, it is desired to execute the modes as easily as possible.